1. Field of the Invention
The present invention relates to image recording methods, and in particular, to an image recording method in which the upper and lower bands of a recording head are overlapped for recording, and recording data for a plurality of dots in the overlapping recording area are assigned to the upper and lower bands when recording is performed.
2. Description of the Related Art
In general, thermal transfer printers are widely used as output apparatuses for computers, word processors, etc., because of high recording quality, low noise, low cost, and ease of maintenance.
In these typical thermal transfer printers, paper is supported in front of a platen, and a heating-element-formed thermal head is mounted on a carriage. While the thermal head and the carriage are reciprocally moved along the platen when an ink ribbon and the paper are supported between the thermal head and the platen, the ink ribbon is reeled out, and power is selectively supplied, based on recording information, to the heating elements, whereby a portion of ink is transferred from the ink ribbon onto the paper so that images such as the desired characters are recorded on the paper.
These thermal transfer printers have problems in that, due to paper feeding errors or the like, an area recorded by one scan of the thermal head, and an area recorded by the next scan are so close to each other that a so-called "black line" is generated, and in that the areas are conversely so far apart that a so-called "white line" is generated.
By way of example, in the case where basic dots comprised of an N.times.M dithering matrix are used as basic cells, and recording is performed at a negative screen angle, with the basic cells used so as to be shifted downward in the scanning direction of the thermal head, when the feeding amount of the paper is approximate, as shown in FIG. 54, the distance between a recording area (upper band) formed by one scan of the thermal head, and a recording area (lower band) formed by the next scan of the thermal head, is uniform.
When the feeding amount of the paper is too small as shown in FIG. 55, the distance between the upper and lower bands narrows to generate a black line. When the feeding amount of the paper is too large, the distance between the upper and lower bands increases to generate a white line. The generated black line and white line are visually undesirable, and greatly affect recording image quality.
Accordingly, in order to prevent black lines and while lines from being generated, a conventional recording method is employed in which the upper and lower bands overlap.
FIG. 57 shows the outline of the conventional recording method. According to the conventional recording method, when a thermal head has, for example, 240 heating dots, recording is performed using an upper band and a lower band to overlap so that the 237th, 238th, 239th, and 240th dots in the upper band respectively correspond to the 1st, 2nd, 3rd, and 4th dot in the lower band. Recording data for the four common dots are assigned to the upper and lower bands.
In assignment of the recording data, in the overlapping recording area (for four bits), while the dots of the thermal head in the arrangement direction (column direction) are being identified, recording data are assigned for each column to the upper and lower bands. In other words, in the case where all four dots in the first column have recording data, as shown in FIG. 57, among the data in the four dots, the first and second dot data are assigned to the upper band, and the third and fourth dot data are assigned to the lower band. In the second column, the first dot data is assigned to the upper band, and the third and fourth dot data are assigned to the lower band. In the third column, the first dot is assigned to the lower band.
As described above, the conventional recording method performs recording in an overlapping recording area by appropriately assigning, for each column, recording data in the dots of a thermal head to upper and lower bands, and driving, based on the assigned recording data, heating elements as the dots.
The conventional thermal transfer printers have a problem in that the processing amount of recording data is extremely large, substantially delaying processing speed since the recording data in dots are assigned for each column. Therefore, in recording that must process a large amount of recording data, such as the case where multigradation recording is performed by controlling, for example, a time for supplying power to the heating dots of a thermal head, and changing the diameter of each dot, a further problem occurs in that high speed recording cannot be performed because the processing of the recording data is insufficient so that recording speed is reduced.